1. Field of the Invention
The present invention generally relates to a dielectric fluids for capacitors and, in particular, to such fluids that are used in capacitors having metallized electrodes formed on plastic film.
2. Statement of the Prior Art
Metallized plastic film capacitors which are used under high stress conditions generally suffer from failures due to electrical or corona discharges between the metallized electrodes. As these discharges cannot be prevented, it is desirable to avoid their harmful effects to thereby improve the operating life of the capacitors. Generally, metallized film capacitors have the advantage of being "self-healing." This means that corona discharges between the electrodes causes part of the electrode to be oxidized or burnt away thereby increasing inter-electrode spacing to thereby stop the arcing. The presence of either air or dielectric liquids between the electrodes only increases the incidences of arcing and generates harmful by-products which further speed degredation of the capacitor. Degradation is also caused by erosion of the electrodes resulting in loss of capacitance. Also, swelling of the polypropylene film by various dielectric liquids causes a loss in the electrical strength of the polypropylene film thus increasing the amount of inter-electrode arcing. For these reasons, it is generally desirable to exclude both air and dielectric fluid from the capacitor roll of metallized plastic film capacitors and to reduce the amount of imbibtion of the dielectric liquid into the plastic film.
It is known generally that air may be excluded from a plastic film capacitor roll by using biaxially oriented polypropylene film which is wound tightly onto a hard core and heated to elevated temperatures for a given period of time. The heat treatment causes shrinkage predominantely in the roll wound direction, resulting in the elimination of air pockets from the roll. Although this process does appear to eliminate the presence of air, it does not necessarily exclude dielectric fluid from entering the capacitor roll. Again, this dielectric liquid may be absorbed into the polypropylene film to reduce its electrical strength and cause undesirable by-products in the case of an electrical discharge taking place in the presence of the liquid. For this reason, means are needed to prevent impregnation of the roll by dielectric fluid and also to prevent imbibtion of the dielectric fluid into the plastic film.